1. Field of the Invention
The present invention relates to a particle containing a hydrophobic dye having a cyanine structure, and to a contrast agent containing the particle.
2. Description of the Related Art
In recent years, a fluorescence imaging method or a photoacoustic imaging method has attracted attention as an imaging method which allows non-invasive diagnosis.
In the fluorescence imaging method, a fluorescent dye is irradiated with light and fluorescence thus emitted from the dye is detected. It is widely used in various types of imaging. In the photoacoustic imaging method, a molecule as an object to be measured is irradiated with light to allow a heat release due to volume expansion of a molecule of the irradiated object, and an intensity and generation position of an acoustic wave resulting from the volume expansion is detected, to thereby obtain an image of the object to be measured. In the fluorescence imaging method or photoacoustic imaging method, a dye may be used as a contrast agent for increasing an intensity of fluorescence or an acoustic wave from a site to be measured.
Indocyanine green (hereinafter may be abbreviated as ICG) is known as a dye to emit an acoustic wave due to light absorption. It should be noted that the ICG as used herein refers to a compound having a cyanine structure and having a structure shown below.

In this regard, however, H+ or K+ as well as Na+ may be used as a counter ion.
In this connection, Journal of Photochemistry and Photobiology B: Biology, 74 (2004) 29-38 (hereinafter, abbreviated as Non Patent Literature 1) discloses an ICG-containing poly(lactide-co-glycolide) acid (hereinafter may be abbreviated as PLGA) particle obtained by an emulsion solvent diffusion method using polyvinyl alcohol (PVA) as a surfactant.
The ICG-containing particle disclosed in Non Patent Literature 1 involves a problem in that, since the ICG is a dye having a hydrophilic functional group, in an aqueous solution such as serum, the ICG may leak out of the particle.